In internal combustion engines, catalytic aftertreatment of the exhaust gases has become widely established for the purposes of complying with legally prescribed emissions values. In order to increase efficiency, modern internal combustion engines often operate with lean fuel-air mixtures with an excess of oxygen. Nitrogen oxides that are generated cannot be reduced during lean-burn operation, because the catalytic reduction of said nitrogen oxides is possible only during rich operation. Therefore, during lean-burn operation, the nitrogen oxides in the exhaust gas are temporarily stored in a NOx trap catalyst, also referred to as lean NOx trap (LNT). If the capacity of the LNT is exhausted, a cycle with a rich exhaust-gas mixture, or substoichiometric operation (λ<1), is performed for the purposes of regenerating the LNT. Such a regeneration is also referred to as rich purge. The aim of said cycle is to reduce the temporarily stored nitrogen oxides.
For the reduction of the nitrogen oxides, use may also be made of a nitrogen oxide reduction catalyst (hereinafter also referred to as catalyst for selective catalytic reaction, or SCR catalyst). A reducing agent is added to the exhaust gas. In general, as reducing agent, an aqueous urea solution is introduced into the exhaust tract upstream of the nitrogen oxide reduction catalyst. A nitrogen oxide reduction catalyst can store a certain amount of ammonia. If the storage function is exhausted, ammonia can escape from the catalytic converter in the event of overdosing. This phenomenon is also referred to as ammonia slippage. Use is also often made of two nitrogen oxide reduction catalysts, of which the first is an active nitrogen oxide reduction catalyst, for which a reducing agent is introduced into the exhaust tract directly upstream, and the second is a passive nitrogen oxide reduction catalyst, for which no reducing agent is introduced into the exhaust tract directly upstream.
The use of two SCR catalysts has numerous advantages. Firstly, the two SCR catalysts often function at different temperatures, such that a larger temperature window can be utilized. Here, the first SCR catalyst operates at a higher temperature than the second, giving rise to an altogether larger temperature window for the reduction of nitrogen oxides. Furthermore, the storage capacity of the first SCR catalyst is limited, for example because it is often the case that a particle filter is combined with the SCR catalyst; a second SCR catalyst thus permits a more effective removal of nitrogen oxides from the exhaust gas. Thirdly, a lower temperature minimizes the aging of the second SCR catalyst, wherein the aging would be manifested in greater ammonia slippage. A certain slippage through the first SCR catalyst is however desired in order that the second SCR catalyst also receives ammonia that it requires in order to reduce nitrogen oxides.
If a low-pressure EGR system (LP EGR, hereinafter referred to for simplicity as EGR) branches off from the exhaust tract, there is however the problem that ammonia passes to the internal combustion engine, and is oxidized there to form nitrogen oxides. This would, in a counter-productive manner, increase the nitrogen oxide content in the exhaust gas. It is therefore the object to as far as possible prevent the loss of ammonia via the EGR, and in the process supply sufficient reducing agent to the second SCR catalyst.
In one example, the issues described above may be addressed by an arrangement in a motor vehicle having an internal combustion engine with an exhaust tract from which a low-pressure exhaust-gas recirculation system branches off and in which an exhaust-gas aftertreatment system is arranged, the exhaust-gas aftertreatment system comprising a diesel oxidation catalyst, a first selective catalytic reduction device downstream of the diesel oxidation catalyst and upstream of an intersection in the exhaust tract from which the low-pressure exhaust-gas recirculation branches off, a second catalytic reduction device located in the exhaust tract downstream of the intersection; at least one first reducing agent feed device which is arranged upstream of the first selective catalytic reduction device and downstream of the diesel oxidation catalyst; and at least one second reducing agent feed device which is arranged downstream of the branching point of the exhaust-gas recirculation system and upstream of the second catalytic reduction device. In this way, the fraction of ammonia in the recirculated exhaust gas is reduced in relation to conventional arrangements, and in this way, additional nitrogen oxide production as a result of the engine-internal combustion of recirculated ammonia is limited. Owing to the presence of a second reducing agent feed device, the ammonia storage by the nitrogen oxide reduction catalysts may be increasingly controlled. Depending on driving conditions and temperature conditions, the amounts of reducing agent introduced can be varied between the first and the second reducing agent feed device. Since it is thus possible in many cases to eliminate significant ammonia slippage from the second catalytic converter, the arrangement of a second reducing agent feed device makes it possible to continue to operate the second catalytic converter in an advantageous manner without the need for conducting ammonia onward, and to nevertheless permit the reduction of nitrogen oxides by way of the third catalytic converter.
In the description, the terms reducing agent and ammonia are used synonymously. In particular, reducing agent is spoken of when referring to the feed into the exhaust tract, because the reducing agent is generally an aqueous urea solution, in particular the commercially available AdBlue®, which is hydrolyzed in the exhaust tract or catalyst to form ammonia and carbon dioxide. The reducing agent itself is however ammonia, which may possibly also be introduced directly into the exhaust tract.
As one example, the exhaust-gas aftertreatment system comprises multiple catalytic converters. The catalytic converters have in each case at least one catalyst, though may in each case also have multiple catalytic converters or exhaust-gas aftertreatment devices such as particle filters, in particular diesel particle filters. A catalyst, in particular an SCR catalyst, may in this case also be in the form of a coating of a particle filter and applied to said particle filter.
The first reducing agent feed device and/or the second reducing agent feed device is preferably designed for introducing gaseous ammonia into the exhaust tract. The second reducing agent feed device is particularly preferably a fourth catalytic converter which has at least one NOx trap catalyst. In this, ammonia can be formed from nitrogen oxides during a regeneration (rich purge). Said ammonia is then received, stored, and used for the reduction of nitrogen oxides, by the nitrogen oxide reduction catalyst that is arranged downstream in the third catalytic converter.
This embodiment of the fourth catalytic converter may make it necessary for fuel to be introduced into the exhaust tract upstream of the fourth catalytic converter, wherein then, the hydrocarbons contained in the fuel are utilized for effecting the regeneration (rich purge) of the fourth catalytic converter or of the NOx trap catalyst contained therein, during which ammonia is formed which can advantageously be utilized as reducing agent. It is therefore preferable if, in the arrangement according to the present disclosure, a feed device for fuel is arranged upstream of the fourth catalytic converter. In a particularly preferred embodiment, the feed device for fuel is an external fuel injector or a so-called vaporizer.
It is furthermore preferable if, in the arrangement, at least one nitrogen oxide sensor is arranged downstream of the third catalytic converter for the purposes of advantageously detecting a nitrogen oxide concentration in the exhaust tract.
A third aspect of the present disclosure relates to a controller storing instructions in non-transitory memory that when executed enable the controller to implement a method for controlling exhaust-gas purification by way of an arrangement according to the present disclosure, having the steps operating the internal combustion engine such that exhaust gas is conducted through the exhaust tract, introducing a reducing agent into the exhaust tract upstream of the second catalytic converter by way of the first reducing agent feed device, conducting nitrogen oxides not reduced in the second catalytic converter in the exhaust-gas flow onward to the third catalytic converter if said nitrogen oxides are not recirculated with the exhaust gas back to the internal combustion engine via the exhaust-gas recirculation arrangement, and introducing a reducing agent downstream of the branching point of the low-pressure exhaust-gas recirculation system and upstream of the third catalytic converter by way of the second reducing agent feed device.
In an exemplary embodiment, in the method, the fourth catalytic converter corresponds to the second reducing agent feed device, and the reducing agent that is introduced is gaseous ammonia produced by the fourth catalytic converter. However, the inventors herein have recognized potential issues with such systems.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.